This application claims the priority of Korean Patent Application No. 02-52461, filed Sep. 2, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical head for recording information on and reading information from an optical disc, and more particularly, to an integrated optical head having an input coupler and an output coupler, the couplers which use an evanescent coupling effect or a light diffraction effect.
2. Description of the Related Art
FIG. 1 is a schematic view of a conventional optical head assembly. Referring to FIG. 1, a conventional optical head assembly includes a laser diode 1, a collimating lens 5, a beam splitter 6, an objective lens 7, and an optical detector 8.
A laser beam radiated from the laser diode 1 is collimated by the collimating lens 5. Then, the collimated beam passes through the beam splitter 6 and is focused by the objective lens 7 so as to form a spot 4 on an optical disc 3. Part of incident beam reflected by the optical disc 3 goes back into the beam splitter 6. Then, the optical path of the reflected beam is changed by 90 degrees by the beam splitter 6 so that the reflected beam is incident upon the optical detector 8. Thereafter, the optical detector 8 converts the incident beam into an electrical signal. A differential amplifier 9 detects a difference between the output signals of two sections into which the optical detector 8 is divided, and produces a tracking error signal.
FIGS. 2A and 2B show an optical head assembly disclosed in the U.S. Pat. No. 4,779,259. The optical head assembly includes an optical integrated circuit (IC) in which optical elements are integrated by a single etching process in order to achieve a light, compact optical head assembly.
Referring to FIG. 2A, a buffer layer 42 is formed on a substrate 41 by oxidation or vapor deposition, and a light guiding layer 43 is formed as a dielectric thin film on the buffer layer 42 by vapor deposition or the like. A collimating lens 45, a beam splitter 46, and a converging-type grating coupler 47 are installed on the light guiding layer 43 by photolithography, electron beam drawings, or plasma etching. The converging-type grating coupler 47 has a plurality of unequally spaced curves. Reference numeral 44 denotes a laser diode to radiate a laser beam, and reference numeral 48 denotes an optical detector having two light receiving elements. The optical detector 48 is attached to one edge of the optical head assembly such that its center is aligned with the optical axis of incident light. Reference numeral 49 denotes an optical disc, and reference numeral 50 denotes information-recorded pits on the optical disc.
A laser beam emitted from the laser diode 44, attached to another edge of the optical IC-type head assembly, passes through the light guiding layer 43 and is collimated by the collimating lens 45. The collimated beam passes over the light guiding layer 43 and is then focused by the grating coupler 47 to form spots on the information pits 50 on the optical disc 49. Part of the incident beam reflected by the optical disc 49 goes back into the grating coupler 47 and passes over the light guiding layer 43 toward the beam splitter 46. The beam incident upon the beam splitter 46 is reflected so as to go toward the optical detector 49.
FIG. 2B shows another embodiment of the optical head assembly of U.S. Pat. No. 4,779,259. The same reference numerals as those of FIG. 2A denote the same elements. Reference numeral 52 denotes a beam splitter located between the laser diode 44 and the grating coupler 47. Reference numerals 541, 542, 543, and 544 denote the detection surfaces of an optical detector built in the light guiding layer 43.
A laser beam emitted from the laser diode 44 passes through the light guiding layer 43 and is focused by the grating coupler 47 to form a spot on the surface of the optical disc 49. Part of the incident laser beam reflected by the optical disc 9 passes through the grating coupler 47 and is divided into two beams by the beam splitter 52. Then, the two beams are directed toward and received by the detection surfaces 541, 542, 543, and 544 of the optical detector.
In such an optical IC-type head assembly, an input coupling efficiency between a laser diode and a light guiding layer and an output coupling efficiency between the light guiding layer and an optical disc are degraded,.such that the output intensity of light does not reach the intensity for recording and reproducing information. Also, since a grating coupler has a small number of apertures, it provides a low focusing intensity. In addition, the detection sensibility of an optical detector with respect to a variation in the wavelength of light reflected by the optical disc is deteriorated.